A mail from Michael Peddemors of linuxmagic.com made me look at a relative new and free DNSBL offering service – http://www.spamrats.com/

They offer 3 types of rbls viz.

1) RATS-Dyna (accessed via. ‘dyna.spamrats.com’) – This lists IPs which are known to make too many SMTP connections or trying too many invalid users and also IPs having rDNS entry which looks like it is a dynamic or home connection. Some IPs it blocked at my server were having rDNS as,
187-35-24-158.dsl.telesp.net.br
ppp-124-121-80-22.revip2.asianet.co.th
host84-240-dynamic.6-79-r.retail.telecomitalia.it

RATS-Dyna was listed as last rbl in my mail server after checking connections against having no rDNS, dnsbl-1.uceprotect.net and zen.spamhaus.org and it blocked surprisingly 10% more connections. In a way, it blocked 10% of probable spam which could have passed through other filters.

2) RATS-NoPtr (accessed via. ‘noptr.spamrats.com’) – This lists IPs which are known to make too many SMTP connections or trying too many invalid users and also IPs having no rDNS entry

What is use of RATS-NoPtr when most mail servers have inherent capability to reject IP with no rDNS entry. Michael Peddemors imply says “Performance actually, hard to believe ” and he might be right.

3) RATS-Spam (accessed via. ’spam.spamrats.com’) – This is an aggressive list and should be used with caution. This lists compromised servers, hosts, or open relays. Make sure you exempt your own customers from this list before using.
They say on their website – “RAT-Spam is better used in a scoring environment (like with spam assassin), rather than a blocking environment”.

For more spam stats see,
http://www.shantanukulkarni.org/rbl-compare.html
http://www.shantanukulkarni.org/spam_analysis.html